1. Technical Field of the Invention
The present invention relates to magnetic resonance imaging including non-contrast angiography, that uses a preparation scan to optimize a desired pulse sequence image quality parameter incorporated in an imaging scan in order to provide improved quality MR images.
2. Related Art
Magnetic resonance imaging (MRI) is a technique of applying a radio-frequency (RF) signal at a Larmor frequency to an object so that nuclear spins positioned in a static magnetic field are magnetically excited and then reconstructing an image from MR signals induced in response to the excitation.
However, it is not always true that an MR imaging scan is performed under the best imaging conditions. That is, an imaging scan is frequently performed under an unfavorable condition where one or more pulse sequence parameters of the imaging scan, which can significantly influence MR image of quality, has not been optimized.
One of the imaging techniques that has recently been highlighted is, for example, non-contrast MR angiography. This imaging technique provides images of blood vessels and/or flows of blood within an object, with no contrast agent administered into the object. In performing non-contrast MR angiography, a three-dimensional imaging scan is preferable when it is desired to obtain more angiographic information.
Such non-contrast MR angiography typically includes, for example, a dephase pulse to suppress a flow void phenomenon. When the flow void phenomenon is generated in response to a flow of blood, some drawbacks, such as a decrease in the intensity of an echo signal to be acquired, are caused. It is therefore preferred that the degree of such flow void phenomenon be detected beforehand, and that imaging conditions be determined for every object to be imaged in consideration of the flow void phenomenon.
In cases where non-contrast MR angiography is directed, for example, to the iliac artery, speeds of blood flowing in the iliac artery are not only different depending on individuals but also substantially different between a healthy person and a non-healthy patient. Even if the iliac artery of the same person is being imaged, speeds of blood flows change depending on which region is to be scanned.
However, from a historical viewpoint, optimization of various parameters of an imaging scan pulse sequence for such phenomena, which should be carried out prior to the imaging scan, has not been adequately studied. Hence it has been difficult for an operator to imagine the inside of an object to be examined, and then to recognize the degree of expected flow void with accuracy in a desired readout direction, before carrying out a three-dimensional scan. Instead, operators typically infer the degree of flow void using their own experience or by trial and error and then try to reflect the inferred degree into imaging conditions. A trial scan could be carried out for inferring the degree of flow void, but this trial scan would not be quantitative. Thus the total imaging time necessary for each person to be examined is, thereby a patient throughput being reduced.
In performing non-contrast MR angiography, image quality parameters of an imaging scan pulse sequence typically may include, in addition to one identifying a degree of flow void, an effective echo time TEeff, a bit indicating flow compensation, inversion recovery time, echo train spacing (ETS), the flip angle of a fat suppression pulse, an inversion time TI after application of a fat suppression pulse, the flip angle of an MT (magnetization transfer) pulse, and the flip angle of a refocusing pulse.
One conventional scan technique is known by a Japanese Patent Laid-open Publication No. 1999-239571. This reference shows an imaging scan using electrocardiographic (ECG) gating, in which a scan to measure an optimized delay time for ECG gating is proposed. This technique, however, takes only ECG gating timing into account, so that this way of scanning is far from providing other various scan parameters.
The present invention, which has attempted to break through the foregoing current situations, provides both a magnetic resonance imaging system and a magnetic resonance imaging method for magnetic resonance imaging with our without an MR contrast agent, the system and method being able to give an optimum value to a desired an imaging scan parameter in a steady and reliable manner, before carrying out the imaging scan.
In order to achieve the above object, as one aspect of the present invention, there is provided a magnetic resonance imaging system for performing an imaging scan based on a desired pulse sequence in order to obtain MR images at a desired region of an object to be imaged. The system comprises a preparation scan performing unit configured to perform a preparation scan to acquire data for a plurality of preparatory images at a common volume in the desired region of the object, the preparation scan being performed with an amount of a desired image quality (e.g., contrast-determining) parameter of the pulse sequence being changed for each of plural preparatory images; a preparatory image producing unit configured to produce the plurality of preparatory images from the data acquired by the performance of the preparation scan; a displaying unit configured to display the plurality of preparatory images; a selection unit configured to allow a desired preparatory image to be selected from the plurality of preparatory images displayed; and a setting unit configured to set, into the imaging scan, the amount of the desired image quality (e.g., contrast-determining) parameter given from the selected preparatory image.
Accordingly, the magnetic resonance imaging system employs the technique of performing the preparation scan to determine an optimum amount of one or more parameters chosen from the image-quality parameters of the imaging scan. Echo data into which changed amounts of the desired parameters are reflected are acquired by the preparation scan, and images produced from the acquired echo data provide an operator with an optimum amount of the desired parameters in the actual imaging scan. This way enables both of MR imaging and MR angiography (with or without an MR contrast agent) to provide images with excellent contrast, less noise, and higher quality depictions.
Preferably, the preparation scan performing unit is configured to perform the preparatory scan with a series of acquisitions. In the preparatory scan the image matrix size is smaller than that of an MR image acquired through the imaging scan. Still more preferably, the preparation scan is set to a two-dimensional scan and the imaging scan is set to a three-dimensional scan.
It is preferred that the preparation scan performing unit is configured to perform the preparation scan prior to the performance of the imaging scan.
It is also preferred that the system further comprises a breath-hold instructing unit configured to instruct the object to continue holding breath of the object during each period of both of the preparation scan and the imaging scan.
By way of example, the pulse sequence is made of a train of pulses belonging to SSFP (Steady State Free Precession)-system pulse sequences.
The desired parameter of the pulse sequence is, for example, at least one parameter selected from a group of parameters composed of: a strength of a pulse to suppress a flow void phenomenon of a fluid of the object; an effective echo time TEeff concerning behaviors of spins of the object; a pulse to compensate spin movements due to flow of fluid in the object; a TI (inversion time) of spins observed when an inversion pulse is applied to the object; an ETS (echo train spacing) time given to echo signals acquired from the object; a flip angle of a fat suppression pulse applied to suppress signals from being acquired from fat of the object; a TI (inversion time) time observed when a fat suppression pulse is applied to the object; a strength of an MT (magnetization transfer) to cause an MT effect resultant from behaviors of spins in the object; and an angle of a refocusing pulse to reduce an MT effect in the object.
As another configuration according to the above aspect, a magnetic resonance imaging system is provided particularly to, a desired parameter, a TI (inversion time) time incorporated in the pulse sequence. The TI time is observed when an inversion pulse is applied to the region of the object. Accordingly, through the preparation scan, an optimized period of the TI time, that is, the amount of the desired parameter, is determined and reflected into the pulse sequence used by the imaging scan.
Still, as another example according to the above aspect, a magnetic resonance imaging system is provided for performing a three-dimensional imaging scan based on a desired pulse sequence in order to obtain an MR image at a desired region of an object to be imaged. The system comprises a preparation scan performing unit configured to perform a two-dimensional preparation scan to acquire data for a plurality of preparatory images at the desired region of the object, the preparation scan being performed with an amount of a desired parameter of the pulse sequence changed every preparatory image; a preparatory image producing unit configured to produce the plurality of preparatory images from the data acquired by the performance of the preparation scan; and a setting unit configured to set the amount of the desired parameter of the imaging scan on the basis of the plurality of preparatory images produced.
Furthermore, as another aspect, there is provided a method of optimizing a parameter of a pulse sequence used by an imaging scan in magnetic resonance imaging. The imaging scan is performed to provide an MR image at a desired region of an object to be imaged. The method comprises the steps of: performing a preparation scan to acquire data for a plurality of preparatory images at the desired region of the object, the preparation scan being performed with an amount of a desired parameter of the pulse sequence changed every preparatory image; producing the plurality of preparatory images from the data acquired by the performance of the preparation scan; displaying the plurality of preparatory images; allowing a desired preparatory image to be selected from the plurality of preparatory images displayed; and setting, into the imaging scan, the amount of the desired parameter given from the selected preparatory image.
Like the above, the method can be provided so as to comprising the steps of performing a two-dimensional preparation scan to acquire data for a plurality of preparatory images at the desired region of the object, the preparation scan being performed with an amount of a desired parameter of the pulse sequence changed every preparatory image; producing the plurality of preparatory images from the data acquired by the performance of the preparation scan; and setting the amount of the desired parameter of the imaging scan on the basis of the plurality of preparatory images produced.